In programs of special analysis of rock samples or core samples taken from a medium, such as an underground region, establishment of initial representative water saturation plays a key role in core sample preparation. The point is to establish fluids in proportions representative of those originally present in the reservoir region after migration of the oil. Typically, if the effectiveness of water injection is to be studied and the capillary pressure curve and relative permeability curve are to be measured, the initial saturation Swi is important and must be representative of the in-situ conditions. The term “initial” is used advisedly here to avoid any confusion with the term “irreducible” which describes the asymptotic saturation obtained with a high capillary pressure for a given set of fluids. In a transition zone, these two saturations are very different.
According to a standard procedure, the samples are extracted from full-diameter core samples, and then cleaned with appropriate solvents. The samples are then brought to initial saturation (Swi) or irreducible saturation (Swirr) depending on their position in terms of capillary pressure, and aged with crude oil. At this stage, the amount of water present also plays a decisive role in obtaining a representative state of wettability. This is why a substantial effort is generally made to establish this initial saturation (Swi).
Several known techniques enable this condition to be reached. For example, the water-saturated sample can be confined in a cell and this water can be displaced by injecting oil. It is known, however, that it is difficult to obtain low water saturations, essentially because of the presence of heterogeneities despite the use of viscous oil (typically 50 cP). Viscous oil can also be somewhat impractical for low permeabilities. The average saturation can hence still be high after breakthrough and residual production can be significant and take several days. Moreover, the saturation profile is highly nonuniform, as is the case with standard centrifugation. This profile can however be reduced by reversing the injection direction. Although centrifugation is the most effective technique for saturating core samples, it cannot be used to establish saturation (Swi) because of the presence of a high saturation profile which can give rise to interpretation problems in later injection experiments. For example, one may cite the substitution technique, the drainage method for which the main difficulty is controlling or imposing the salinity and the saturation profile. The ideal would be to use a capillarity displacement process, as is the case in situ, with an experimental time compatible with the schedule of the development or evaluation program, which is generally short. To avoid non-uniform profiles and obtain low water saturation, the porous plate method can be used instead of the above-mentioned techniques. The experiments are time-consuming, taking a few weeks to a few months, particularly in the case of long core samples. Moreover, the capillary contact between the core sample and the porous plate is often difficult to optimize and may lead to a low success rate.
The centrifugation technique is probably the most attractive solution. This is a displacement process dominated by capillarity, which is rapid and inexpensive and has a number of practical advantages. Its main drawback is however the non-uniformity of the saturation profile and, for certain centrifuges, the limit imposed by the length of the core samples.
Furthermore, gas permeability measuring apparatuses are generally known in the prior art. Such apparatuses typically include one or more sensing heads which are adapted for holding a membrane material across a chamber, wherein a gas such as oxygen may be admitted into the chamber on one side of the membrane, and a detector such as an oxygen detector may be coupled via passages to the other side of the chamber, to measure the amount of oxygen which passes through the membrane. Since all membranes are permeable to some extent, it is usually possible to detect a measurable amount of oxygen passing through the membrane over a finite period of time. In the prior art, gas permeability measuring apparatuses utilized one or more of such measuring heads coupled via hoses and tubing to sensors and the like, to perform fairly accurate measurements of membrane permeability.
Measurement of gas permeability through membranes requires extremely sensitive gas detectors or sensors, for the quantities of measured gas are frequently quite low. It is therefore extremely important that the entire system involved in such measurements be maintained under tightly sealed conditions, particularly with respect to all of the gas flow passages leading to the gas detector. Prior art permeability measuring instruments typically utilize hoses or tubing to interconnect the necessary instrumentation, wherein each of the connecting junctions is susceptible to leakage.
Therefore, it would be advantageous to provide a permeability measurement apparatus and method to overcome the above short comings.